The star J0815+4729 is one of the most iron-poor known stars ([Fe/H]=−5.49), and its chemical composition is considered a fingerprint of nucleosynthesis in the first stars. Nearby in the archive are works on lithium abundances, organic molecules in space, planetary disks, and neutron stars, but there are almost no direct neighbors on this topic: this work stands apart. Until now, carbon abundances in such objects have been determined almost exclusively from molecular CH bands, because the atomic C I lines are too weak. J0815+4729 is unique in that its extreme carbon and oxygen enrichment makes the atomic C I and O I lines measurable, allowing the reliability of the CH indicator to be tested for the first time.
Based on high-resolution spectra from the Keck I telescope (R~37 500), new models of carbon and oxygen atoms were constructed, and an analysis was performed under local thermodynamic equilibrium (LTE) and non-LTE (NLTE) conditions. For the atomic C I lines, a line-by-line fitting was used; for the molecular CH and C2 bands, chi-square minimization against synthetic spectra. The stellar atmospheric parameters were determined independently from hydrogen and iron lines: effective temperature 6050 K, surface gravity log g=4.6, metallicity [Fe/H]=−5.49.
The carbon abundance from C I lines in NLTE is A(C)=8.08±0.07, which agrees with the result from C2 lines (A(C)=7.96±0.08 in LTE), but is 0.6 dex higher than the value from CH lines (A(C)=7.47±0.17). The discrepancy cannot be explained by changing the temperature within reasonable limits, nor by 3D effects, which only increase the difference. It may be partly due to NLTE effects for molecular lines or to systematic errors in the crowded G-band region of CH. The oxygen abundance from the infrared O I triplet is A(O)=7.25±0.11 in NLTE, and the carbon-to-oxygen ratio is [C/O]=1.06±0.11.
This work uses atomic C I lines for the first time for a star with [Fe/H]<−5 and shows that the traditional CH indicator systematically underestimates the carbon abundance by about 0.6 dex. This casts doubt on quantitative conclusions about chemical evolution based on CH measurements in the most primitive stars and may change the classification of such objects as CEMP-no or CEMP-s. Unlike neighboring works, where CH remained the only available indicator, here a cross-check with three independent methods was possible.
The topic could develop toward detailed modeling of NLTE effects for molecular CH and C2 lines in extremely metal-poor stars, as well as toward a systematic revision of carbon abundances in known hyper-iron-poor stars using atomic C I lines where possible. New observations with higher signal-to-noise ratios and possibly 3D atmospheric models will be required.
The results will affect the interpretation of the chemical composition of the oldest stars and models of nucleosynthesis in the first supernovae, as well as the classification criteria for CEMP stars.
Conduct a similar analysis for other stars with [Fe/H]<−4.5 where C I lines are available, and develop reliable NLTE models for the molecular CH and C2 bands under extremely low metallicity conditions.
This work is directly related to the unresolved problem of determining carbon abundances in the most primitive stars: systematic discrepancies between the CH, C2, and C I indicators call into question the reliability of existing [C/Fe] estimates and, consequently, conclusions about the nature of the first stars and the sources of enrichment in the early Galaxy.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
For the star J0815+4729 with [Fe/H]=−5.49, the carbon abundance was measured by three independent methods: atomic C I lines, molecular C2 lines, and CH lines. The atomic C I and C2 lines give consistent values around A(C)=8.0, while CH indicates a value 0.6 dex lower. This discrepancy calls into question the use of CH as the primary carbon indicator in the most primitive stars.
Where its strength is
For the first time, atomic C I lines were used for a star with [Fe/H]<−5, allowing a cross-check of three indicators and revealing a systematic underestimation of carbon abundance from CH lines.
Where it could go next
Worth trying
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
J0815+4729 is so carbon-rich that its spectrum shows not only molecular bands but also weak atomic lines of neutral carbon: a rare opportunity for a star with [Fe/H]=−5.49. Among the 16 known stars with [Fe/H]<−4.5, it has the highest carbon abundance.